1. Field of the Invention
This invention relates to a pipe proximity warning device and, more particularly, to a pipe proximity warning device for preventing accidental damage to plastic gas lines.
2. Description of the Prior Art
Before proceeding with any earth digging operation, the operator of a piece of earth moving equipment should determine whether any lines, such as power lines, cable lines, phone lines, water lines, or gas lines are buried within the area of the dig. If the area does contain such lines, the operator should also determine the location of any such lines to prevent the lines from being damaged. Numerous devices have been designed to assist an operator in locating a buried line.
One type of conventional pipeline detector establishes a circuit between the earth moving equipment and the pipeline. This type of detector monitors the established circuit and sounds an alarm or disables the earth moving equipment when the equipment makes contact with the pipeline. Examples of this type of detector are found in U.S. Pat. No. 2,131,291 issued to Maust, U.S. Pat. No. 3,190,476 issued to Smith et al., U.S. Pat. No. 3,911,602 issued to Trevathan, and U.S. Pat. No. 3,916,298 issued to Ulrich. A disadvantage of this type of detector is that some damage may already have occurred to the pipeline by the time the detector signals that the earth moving equipment has contacted the pipeline. Furthermore, with this type of detector, the pipeline to be detected must be formed of a metallic or electrically conductive structure.
Another type of prior art proximity detector employs a transmitting antenna and a receiving antenna to locate underground metallic structures. Some examples of this type of detector may be found in U.S. Pat. No. 3,858,737 issued to Senoo and in U.S. Pat. No. 4,600,356 issued to Bridges et al. While this type of detector is able to detect the pipelines prior to any contact with the pipeline, the detector still requires the pipeline to be formed of a metallic or conducting material. Additionally, this type of detector does not provide an accurate location of the pipeline but instead only indicates the presence of a metallic structure.
Recently, the installation of gas lines has been performed using plastic pipelines. As a result, the conventional proximity sensors which detect metallic pipes are unable to detect the presence of the plastic pipeline. To enable the detection of the plastic pipelines, the plastic lines are generally installed with a metallic trace wire which is placed in parallel and close to the plastic pipeline.
By scanning the area with a hand held instrument, an operator can then estimate the location, course, and depth of the plastic pipelines, The hand held instrument contains a receiver for detecting a radio frequency (RF) signal induced on the metallic trace wire. The instrument, however, only indicates the presence of the trace wire and only provides a rough estimate of the location of the plastic gas lines. The use of the hand held device is often inaccurate, time consuming, and lowers the productivity of the digging operation. Moreover, even with a rough estimate of the wire's location, the gas lines would still become damaged by the backhoe because of the inaccuracy of the instrument and because of errors in judgement on behalf of the backhoe operator.
A system disclosed in U.S. Pat. No. 3,907,136 to Christides et al. may be used to prevent damage to the plastic gas lines. The system comprises a plurality of inductor coils 38 mounted into the teeth 37 of a bucket 18 for detecting a signal on the trace wire buried with the plastic gas line. The system compares signals from adjacent inductor coils 38 and will sound an alarm when the difference between any two adjacent signals is greater than a first threshold level and will also automatically stop the machine when the difference between any two adjacent signals is greater than a second threshold level.
The system disclosed in Christides et al., however, is prone to several disadvantages. For instance, the system is highly dependent upon the magnitude of the signals on the trace wire. When the signals on the wire are relatively large in magnitude, the differences between two adjacent signals from inductor coils 38 will exceed the fixed thresholds at distances further from the wire than when the signals on the wire are relatively small in magnitude. Thus, the system cannot indicate the distance to the wire and cannot provide a warning or stop the backhoe at constant distances to the wire. As a result, the digging may have to be performed by hand sooner than necessary, thereby lowering the efficiency of the digging operation.
In addition to not being able to indicate the distance to the wire, the system is also not able to indicate the location of the wire. Consequently, when the system provides a warning, the operator does not know whether the wire is located directly beneath the bucket 13 or at any other angle into the surface of the earth. The efficiency of the digging operation is therefore lowered due to the unknown direction of the trace wire.
Thus, there is a need in the prior art for a pipe proximity warning device that accurately detects the presence and location of plastic gas lines. There is also a need in the prior art for a pipe proximity warning device that indicates the distance and direction from the earth moving or other equipment to a plastic pipe and a device that warns an operator that the digging operation is approaching the plastic pipe. Additionally, there is a need in the prior art for a pipe proximity warning device that need not rely upon a hand held instrument which only roughly locates the pipe.